1. Field of the Invention
The present invention relates to: a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film formed on an amorphous insulating substrate, and a method for producing such a crystalline semiconductor film; and a semiconductor device using such a semiconductor film and a method for producing the semiconductor device. More specifically, the present invention relates to: a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film formed on an amorphous insulating substrate by applying thermal energy, wherein the thermal energy is applied by irradiating the amorphous semiconductor film with laser light, and a method for producing such a crystalline semiconductor film; and a semiconductor device using such a semiconductor film and a method for producing the semiconductor device.
2. Description of the Related Art
In recent years, active matrix type liquid crystal display apparatuses have been receiving attention as being advantageous, i.e., they are thin, light-weight, and low power consumption displays. In the art of active matrix type liquid crystal display apparatuses having such advantages, for the purpose of fulfilling various demands, such as an increase in display size, an increase in display resolution, a reduction of production cost, etc., a technique for forming a thin film transistor (hereinafter, referred to as “TFT”), as a liquid crystal driving element, using a polycrystalline semiconductor thin film on a less-expensive, low-melting point glass substrate, has been greatly expected.
As a technique for forming a polycrystalline semiconductor thin film at a low temperature around 600° C., a solid phase growth method is known, wherein after an amorphous semiconductor film is formed on a low-melting point glass substrate, a thermal treatment is performed on the substrate at about 600° C. for several hours to several tens of hours, thereby crystallizing the amorphous semiconductor film.
There is an alternative method for producing a crystalline semiconductor film, wherein an amorphous semiconductor film formed on a low-melting point glass substrate is locally irradiated with laser light, or the like, so that the irradiated film melts due to thermal energy generated from the laser light and changes into a crystalline semiconductor film.
Techniques of crystallizing an amorphous semiconductor film by irradiating laser light or the like have been studied for about 30 years. Among these, the study of a method of irradiating an amorphous semiconductor film using laser light as an energy source was started since the 1980s for production of an SOI substrate. In the 1990s, this method was used during the course of development of a production method of a liquid crystal panel based on a low temperature polysilicon technique, and mass production of such liquid crystal panels. This method can be regarded as the most fruitful method. Especially when using a less expensive substrate which cannot withstand a high temperature process, such as a glass substrate, or the like, it is necessary to apply thermal energy within a very short time period. A thermal energy source usable for such a case is only a pulse oscillation laser.
A polycrystalline semiconductor thin film formed using the above described method is patterned into the shape of islands, which will be active layers of TFTs. After the surface treatment has been performed on the patterned film, a gate insulating film is formed on the film. A method for crystallizing an amorphous semiconductor thin film by irradiating laser light, or the like, requires a relatively short time for crystallization in comparison to a crystallization method using a heating treatment. Furthermore, a polycrystalline semiconductor film formed using this method has high electron mobility in comparison to a polycrystalline semiconductor film formed using a crystallization method which uses a heating treatment.
IEEE Electron Dev. Lett., EDL-7, 276, 1986 (Samejima, et al.; hereinafter, Document 1) discloses a method for crystallizing an amorphous silicon thin film formed on a glass substrate by irradiating the film with a high-output excimer laser. The method for crystallizing an amorphous silicon thin film disclosed in Document 1 was originally conceived by Samejima, et al. In Document 1, the energy density of laser light applied on the amorphous silicon thin film is set to a value such that an upper portion of the silicon thin film partially melts. After the initial study by Samejima, et al., the relationship between the energy density of applied laser light and the size of a crystal grain formed was specifically studied. From such studies, it was found that as the energy density of laser light increases, the size of a crystal grain increases.
Such an increase of crystal grain size is studied in detail in J. Appl. Phys. 82, 4086 (hereinafter, Document 2). Document 2 reports that when the depth to which an amorphous semiconductor film is melted by irradiation with laser light was almost equal to the thickness of the amorphous silicon thin film, i.e., almost reached the interface between the silicon thin film and an underlying substrate, a huge crystal grain having a diameter of about several microns is formed. This is because a crystal grain remaining at the interface between the silicon thin film and its underlying layer serves as a crystal nucleus when initiating crystal solidification, whereby a crystal grain having a large diameter can be grown.
However, in the crystallization method described in Document 2, if the energy density of laser light is high so as to exceed a value at which a melted silicon thin film almost reaches the interface with an underlying layer, so that the silicon thin film is totally melted to the underlying layer, a cooling step suddenly occurs and nuclei are generated randomly. In such a case, a crystal grain formed is very small, or the silicon thin film returns to an amorphous state. Thus, in an actual case, in consideration of a variation in power of laser light, the laser light is set so as to produce energy slightly smaller than an energy density that can totally melt the silicon thin film. Under such a laser light irradiation condition, the diameter of a resultant crystal grain is about several hundreds of nanometers. Currently, mass production based on a low temperature polysilicon production technique is carried out according to the laser light irradiation condition determined based on the study of Document 2. Typical carrier mobility in a TFT produced based on the technique disclosed in Document 2 is 150 cm2/Vs for an n-channel type TFT, and 80 cm2/Vs for a p-channel type TFT.
After a liquid crystal panel having a semiconductor device formed from a polysilicon semiconductor film crystallized by irradiation with pulsed laser light was realized, an increase of performance of the polysilicon semiconductor thin film so as to realize an active matrix TFT substrate on which circuit elements having various functions are integrated has been highly demanded.
However, in the above-described method for crystallizing a semiconductor thin film by irradiation with laser light, or the like, a crystal grain of a crystallized polycrystalline semiconductor thin film is susceptible to the irradiation energy density of laser light or the like, a beam profile, the state of irradiated film surface, etc., and accordingly, it is difficult to form crystal grains over a large area glass substrate so as to have a uniform diameter and crystal orientation. In the case where active layers of TFTs are formed of such a polycrystalline semiconductor thin film where the diameter of the crystal grains is not uniform, the carrier mobility is large in a TFT having an active layer including large crystal grains, and the carrier mobility is small in a TFT having an active layer including small crystal grains. As a result, among a large number of TFTs formed over a single substrate, their characteristics are largely varied. Furthermore, if the crystal orientation is different among crystal grains, the mobility of carriers at an interface portion between the crystal grains is largely decreased.
A study which addresses such a problem proposes a crystallization method wherein an amorphous silicon film is totally melted in a crystallization step using laser light irradiation, and the generation of random nuclei is suppressed in the crystallization step while lateral growth is suppressed, whereby TFT characteristics which are equivalent to those of a monocrystalline substrate.
There are various methods proposed at the stage of study only. These methods are based on a common basic idea that, in a silicon thin film, totally melted regions and partially melted regions are mixedly formed such that these regions are in contact with each other, and crystal nuclei present in the partially melted regions are utilized for initiating crystallization of the totally melted regions. In the description provided below, two methods will be described as examples of a method which can be employed at a practical level.
In a first method, which is described in Appl. Phys. Lett. 69(19), 4 Nov. 1996 (hereinafter, Document 3), some regions in a semiconductor film are irradiated with a laser beam having an extremely high aspect ratio such that the semiconductor film in the irradiated regions are totally melted. Then, lateral crystal growth from crystalline semiconductor components included in non-irradiated film regions, which are adjacent to the irradiated region, is induced. In this method, the laser light scans over the semiconductor film with an interval (i.e., a non-irradiated region between irradiated regions) substantially equal to a distance over which lateral growth can be obtained during a single melting process, whereby crystal grains are grown in one direction along the scanning direction of the laser light. J. Im, et al., named this crystallization method “SLS” (Sequential Lateral Solidification) after such a methodology.
In the case where the first method is employed, crystals having any crystal length can be formed over a film without leaving a gap therebetween.
In a second method, which is described in IEEE Electron Device Meeting, San Francisco (hereinafter, Document 4), an aluminum film is formed in a stripe pattern on an amorphous silicon thin film which serves as a beam reflecting film.
Part (a) of FIG. 7 is a cross-sectional view illustrating a crystal growth method described in Document 4. Part (b) of FIG. 7 is a plan view of a crystalline semiconductor film obtained by this crystal growth method.
In the structure shown in part (a) of FIG. 7, an amorphous silicon thin film 2 is formed on an insulative substrate 1, and a beam reflecting film 3 is provided on the amorphous silicon thin film 2 in a stripe pattern so as to partially cover the upper surface of the amorphous silicon thin film 2. According to the method of Document 4, when the entire structure of part (a) of FIG. 7 is irradiated with a laser beam from the upper side, the amorphous silicon thin film 2 in regions 2a, which are not covered with the beam reflecting film 3, is totally melted (totally melted portions) because the amorphous silicon thin film 2 in regions 2a is exposed to the laser light. In regions 2b covered with the beam reflecting film 3, the amorphous silicon thin film 2 is not melted (non-melted portions) because the laser light does not reach the amorphous silicon thin film 2 in regions 2b. The totally-melted portions and the non-melted portions occur in an alternate order without interruption therebetween (i.e., the totally-melted portions and the non-melted portions are in contact with each other) because the beam reflecting film 3 is provided in a stripe pattern. The melted silicon in the totally-melted portion 2a laterally grows into crystals on the basis of crystal nuclei present in the non-melted portions 2b. As a result, as shown in part (b) of FIG. 7, crystal growth occurs from an interface between the totally-melted portion 2a and the non-melted portion 2b toward a substantially central portion of the totally-melted portion 2a. At the substantially central portion of the totally-melted portion 2a, the crystal grains formed appear as though they are crushed against each other.
AM-LCD 2000 Digest, p. 281 (hereinafter, Document 5), describes a method where a beam reflecting film is not provided, but some portions of a silicon thin film are formed thicker than the other portions. In the method of Document 5, the thinner portions of the silicon thin film result in totally-melted portions, whereas the thicker portions result in partially-melted portions. The melted silicon in the totally-melted portions laterally grows into crystals on the basis of crystal nuclei present in the partially-melted portions.
Japanese Laid-Open Publication No. 2000-260709 (hereinafter, Document 6) describes providing an antireflection film on a semiconductor film, in place of the beam reflecting film on the semiconductor film as in Document 4. In this method, by laser irradiation, regions of the semiconductor film which are covered with the antireflection film result in totally-melted portions, whereas regions which are not covered with the antireflection film result in partially-melted portions. The melted silicon in the totally-melted portions laterally grows into crystals on the basis of crystal nuclei present in the partially-melted portions.
In the above second methods described in Documents 4-6, a reflection (or antireflection) film is patterned on a silicon thin film so as to have a desired pattern, or some portions of the silicon thin film are formed thicker than the other portions according to a desired pattern, whereby the direction in which growth of crystal grains advances (“crystal growth direction”) is determined regardless of the scanning direction of a laser beam. These methods are advantageous in that silicon films having crystal growth directions perpendicular to each other can be formed mixedly on the same substrate.
However, the above first and second methods have the following problems.
In the first method, the length of lateral crystal growth achieved by a single irradiation with laser light is very small, about 0.5 μm to about 4 μm. Because of this lateral crystal growth length, it is necessary to move the laser light so as to scan a semiconductor film with high precision corresponding to the lateral crystal growth length. As a result, processing time for crystallization becomes long. Further, it is necessary to prepare an apparatus having a special structure for providing the laser light in such a precise manner. Furthermore, in the first method, extremely elongated crystal grains extending along the scanning direction of laser light are formed. In the case where a TFT is formed from a crystalline silicon film having such crystal grains, the carrier mobility is extremely high if the direction in which carriers flow is the same as the direction in which the crystal grains extend. However, it is known that, if the direction in which carriers flow is perpendicular to the direction in which the crystal grains extend, the carrier mobility is about ⅓ of that achieved when the direction in which carriers flow is the same as the direction in which the crystal grains extend. In this method, since the direction in which crystal grains extend is determined based on the scanning direction of a laser beam, a high characteristic can be obtained only in one direction. Crystal grains extending different directions cannot be formed mixedly on the same substrate. Thus, this method imposes an undesired restriction on the design of a circuit device.
The second method has a problem that the length of crystal grains is limited by a lateral growth distance achieved by a single laser irradiation. Further, in this crystallization method, it is necessary to form non-melted or partially-melted portions at a predetermined interval so as to be adjacent to melted portions, because crystal growth in the melted portions is initiated by crystal nuclei present in the non-melted or partially-melted portions. However, if a non-melted (or partially-melted) portion formed is too small (narrow) in an attempt to place a laterally-grown crystal grain in a totally-melted portion as close as possible to a crystal grain in an adjacent totally-melted portion which is neighboring via the narrow non-melted portion, an initial nucleus in the non-melted portion, which is used for initiating crystal growth in the totally-melted portions, disappears due to laser light irradiation, and very small crystal nuclei may be formed due to random generation of nuclei. Thus, a non-melted portion has to be formed so as to have a certain size in order to avoid the above problem. Thus, in this crystallization method, the device size may increase because of an area which is required for forming non-melted or partially-melted portions.